Among the polyamides known to date, some exhibit a major advantage as a result of their noteworthy mechanical properties, in particular due to their high tensile and flexural moduli.
Mention may in particular be made of polyphthalamides (PPAs), which are semiaromatic polyamides which exhibit a high tensile modulus of the order of 3 GPa. This is also the case with polyamide MXD.6, the product of the condensation of MXD and adipic acid, MXD denoting meta-xylylenediamine (optionally mixed with a minor amount of para-xylylenediamine or PXD).
While PPAs and MXD.6 are highly satisfactory in terms of mechanical properties (in particular high tensile modulus), they however exhibit two major disadvantages:                First, in view of their high melting point, PPAs and MXD.6 exhibit a high transformation temperature (typically of greater than 280° C.). In addition to being energy-intensive, the use at a high transformation temperature limits the introduction, into the compositions based on PPAs or MXD.6, of certain reinforcers and/or additives which decompose at such temperatures.        Secondly, as a result of their glass transition temperature (Tg), which is also high, and their slow kinetics of crystallization, the shaping of the materials based on PPAs or on MXD.6 requires operating at relatively high mold temperatures, typically of the order of 120° C. to 130° C. (30 to 40° C. above the Tg), in order to obtain maximum crystallization and thus to confer, on the material, optimum mechanical and dimensional stability properties.        
More particularly, during the shaping by injection molding of the materials based on PPAs or MXD.6, it is necessary to use molds using oil as heat-exchange fluid, such molds being more restricting in use and less widespread at molders than molds using water as heat-exchange fluid.
In order to optimize the crystallization of a composition based on MXD.6, the paper entitled Effect of Nucleating Additives on Crystallization of Poly(m-xylylene adipamide) and published in the review Polymer Engineering and Science in 2007, pp. 365-373, provides for the introduction of nucleating agents, in the case in point talc and PA 6.6, the melting point of which is of the order of 250° C.
The polyamide MXD.10, the product of the condensation of MXD and sebacic acid (decanedioic acid), also exhibits good mechanical properties and in particular a high tensile modulus.
As MXD.10 exhibits a lower melting point (of approximately 193° C.) than that of PPAs or MXD.6, its transformation temperature of between 200° C. and 270° C., and more generally of between 210° C. and 260° C., is lower than that of PPAs and MXD.6, which limits the energy consumption. Furthermore, it is possible to envisage compositions based on MXD.10 comprising certain reinforcers and/or additives which decompose at the transformation temperatures of PPAs and MXD.6.
Furthermore, the density of MXD.10 is lower than that generally observed with regard to PPAs or MXD.6. Thus, the objects obtained from compositions based on MXD.10 exhibit the advantage of being lighter, compared with those obtained from compositions based on PPA or on MXD.6.
On the other hand, as in the case of PPAs or MXD.6, a high mold temperature (typically of the order of 120° C.) is necessary during its shaping by injection molding in order to ensure maximum crystallization of the product and thus to confer optimum mechanical and dimensional stability properties.
In order to improve the molding conditions of a composition based on MXD.10, in particular by lowering the duration of the cooling stage during the shaping by injection molding, the document EP 0 272 503 provides for the addition, to a composition comprising 100 parts by weight of polyamide MXD.10, of from 1 to 20 parts by weight of a crystalline polyamide having a melting point greater by approximately 20 to 30° C. than that of MXD.10.
The crystalline polyamide thus behaves as a nucleating agent, in the absence of any other nucleating agent of the inorganic filler type, such as the talc mentioned above.
In the document EP 0 272 503, the best molding conditions, in particular a rapid cycle, are obtained for a mold temperature of 130° C.
Furthermore, the document WO 2011/010039 describes a composition based on polyamide of MXD.10 type having a melting point Tf1, comprising a second polyamide having a melting point Tf2 of between Tf1−40° C. and Tf1+20° C. This second polyamide acts as nucleating agent. It makes it possible to control the crystallization of the composition.
However, there still exists a need to succeed in accelerating the kinetics of crystallization of a composition based on polyamide, and in particular based on MXD.10 or the like, while retaining the mechanical and dimensional stability properties of the crystalline material.